Free cutting alloys



25 such aluminum-copper-silicon alloys.

Patented Apr. 13, 1 93 7 FREE CUTTING ALLOY-S Louis w. Kempf and Walter A. Deanycleveland, Ohio, assignors to Aluminum Company of America, Pittsburgh, Pa., a. corporation of Pennsylvania.

No Drawing.

5 Claims.

' The invention relates to aluminum base alloys and is particularly concerned with aluminum base alloys containing copper and silicon.

Aluminum base alloys containing between 3 per .cent and 12 per cent of copper have been widely used heretofore. The copper imparts good casting characteristics and increases the tensile strength, yield strength, and hardness of the alloy. In the lower portion'oi the copper range the alloy may be mechanically deformed by the well known commercial processes such as rolling, forging, drawing, or extrusion. Alloys containing more than about 6 per cent copper are generally used in the cast condition. Throughout 15 the entire range of 3 to 12 per cent copper, howalloys in' such small amounts as 0.5 to 3 per cent further improves the casting characteristics and, to some extent also, the tensile properties. Our

1 invention which is described herinbelow is dlrected to improving the machining quality of Since all commercial aluminum base alloys contain some silicon as an impurity, usually less than, or not greatly in excess of, 0.5 per cent, it is to be understood that the amount of silicondiscussed 3G herein, and hereinafter claimed, represents the per-silicon alloy. v

There are, however, some applications wherein aluminum-copper-silicon alloys as hereinabove L disclosed might be conveniently and profitably used except for an inherent disadvantage which 40 militates against their-use in the production of certain articles requiring exacting machining operations. Mechanical cutting operations such as drilling, shaping, or lathe-cutting are successfully carried out only by using certain precautions which increase the cost of production and perhapsfavorthechoice of another metal or alloy which can be machined more readily but which is not so desirable in other respects, as for example, in physical properties. When alloys are diflicultto. machine this disadvantage becomes evident, in many cases, through rapid wear of the cutting tool edge, so that frequent tool re-sharpening isrequired. Despite contin ual lubrication the machined sln'face is rough and irregular, and the chip has a tendency to and bismuth free machining elements.

Application December 28, 1955, Serial No. 56.541

form a continuous curl or spiral which often fouls the tool. or the moving parts of the machine, It is immediately apparent that there is need for an alloy of good working characteristics and satisfactory physical properties, yet possessing such favorable machining properties that the complete machining operations may be performed economically and successfully, and may be productive of a pleasing surface appearance.

Accordingly an object of our invention is the provision of an aluminum base alloy containing from about 3 per cent to about 12 per cent of copper, and from about 0.5 per cent to about 3.0 per cent of silicon which may be readily and economically machined.

Our invention resides in the discovery that the foregoing object is effected by the addition or two or more of the elements lead, tin, thallium, cadmium or bismuth. The aluminum base alloys to which these elements are added inthe proportions specified below, are known as free cutting or free machining alloys because they 'can be machined more rapidly than similar alloys without these elements and yet have as good or a better finished surface. series of investigations we have discovered that these five meta-ls when added to aluminum base alloys, form a class of alloying elements by reason of their favorable eiiect upon the machining properties of these alloys. In recognition of this efiect we term lead, tin, thallium, cadmium We have further discovered that the simultaneous presence of two or more of these elements is productive of an improvement in free machining characteristics which is considerably greater than that caused by the presence of the same total amount of a single free machining element. For example, the addition of 0.5 per cent of lead and 0.5 per cent of tin to an aluminum base alloy containing about 5 per cent of copper and 0.75 per cent of silicon effects a greater improvement in machiningquality than does the addition of 1.0 per cent of either lead or tin singly.

These flve elementswe believe are unique with respect to their-eflect on the machining characteristics of aluminum-copper-silicon alloys. It is a fortunate circumstance, therefore,that they are also 0! relatively low melting point, a. fact which makes possible their addition to molten aluminum in the'pure state, without the intervention of so-called rich'alloys. As a matter of fact, we have observed that of all. the metals who e melting point is lower than about 327 0.,

the citing point of lead," the live elementswef After an extended l have selected are the only ones which are commercially suitable and that impart free cutting characteristics but do not have undesirable effect on the fundamental physical properties of the 5 base alloy. r The total amount of-free machining elements should not be less than about 0.05 per cent since below this amount there is scarcely any advantageous effect. We have determined that a maximum limit of about 6 per cent total of two or more of the free machining elements is sumeient ior satisfactory commercial results, since although the free machining effect persists beyond this amount, certain of the other physical properties may be unfavorably affected. Aluminum-copper-silicon alloys containing two or more of the free machining elements lead, tin.

thallium, cadmium and bismuth may be machined more rapidly, with less tool wear, less tool sharpening, better quality ofchip and better machined surface than the same base alloys without the free machining additions, and in fact better than the same, base alloys containing an equivalent total amount of a single free machining element.

Since aluminum base alloys containing from 3 to 12 per cent of copper have a wide variety of applications we list several alloys each of which may be said to be preferred for a particular purpose. As an alloy for mechanical deformation we suggest an aluminum base alloy containing 5 per cent of copper, 0.75 per cent of. silicon and a total of 1 per cent of free cutting constituents,

3 For an alloy with excellent characteristics to be used in the unworked condition we suggest an alloy containing 10 percent of copper, 1.25 per cent of silicon and a total of 3 per cent of free machining elements, the balance being aluminum.

For certain purposes, notably the improvement of tensile strength, hardness and grain structure, the alloys as hereinabove disclosed 'may be improved by the. addition of one or more of the group of elements composed of molybdenum,

vanadium, titanium, tungsten, zirconium, and chromium. From 0.05 to 1 per cent of any one of these elements may be used alone, but if more than one is employed the total amount should not exceed about 2 per cent.

It is characteristic of the live elements lead, tin, thallium, cadmium, and bismuth that they form with aluminum a series of alloys'of limited liquid solubility. We have reason to believe that the free machining elements are the only elements which exhibit this characteristic, with the possible exception of several metals which are not regarded as having any commercial promise as additions to aluminumbase alloys. Within 60 the range disclosed and claimed however the free machining elements may be added without unusual. difilculty. We suspect that this characteristic feature of the disclosed elemts may be one of the significant factors which contribute to the r 65 free machining effect. We believe that this effect is further strengthened bydistributing the free machining constituent relatively homogenewsiy throughout the solid matrix, since these free machining constituents are also practically insoluble in the solid aluminum base. 1

The free machining alloys which have been described hereinabove may be subjected to the thermal treatments well known in the art to imve their strength and hardness.- We have found that a solution heat treatment and subsequentaging does not impair the free machining qualltyot the alloys and'in many instances the treatment even tends to improve this property. For many purposes a relatively high strength and hardness are necessary to the successful performance of the machined article and hence the alloy must be heat treated. This treatment is generally applied prior-to the machining operation.

As herelnabove indicated the free machining elements, by reason of their low melting point. may be added to the moltenaluminum alloy in pure metallic form. However, since some dimculty may be encountered in introducing them in the higher percentages of our disclosed range we prefer to use the method whiclr is more fully described in U. 8. Patent No. 1,959,029, issued March 15, 1934. Briefly it involves heating the melt to a somewhat higher temperature than is customary, and vigorously stirring it in excess of a critical period of time.

The term aluminum as used herein and in the appended claims embraces the usual impurities found in aluminum ingot of commercial grade or picked up in the course of the ordinary handling operations incident to melting practice.

We claim:

1. An aluminum base alloy consisting of about 5 per cent copper, 0.75 per cent silicon, 0.5 per cent lead and 0.5 per cent tin, the balance being aluminum.

2. An aluminum base alloy consisting of about 5 per cent copper, 0.75 per cent silicon, 0.5 per cent bismuth and 0.5 per cent cadmium, the bal- -ance being aluminum.

' 3. A free cutting alloy containing from 3 to 12' per cent of copper, from 0.5 to 3 per cent of silicon and a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, tin, thallium, cadmium, and hismuth, to improve its machining properties, the balance being substantially aluminum.

4. A free cutting alloycontaining from 3 to 12 per cent of copper, from 0.5 to 3 per cent of silicon, from 0.05 to 2 per cent of hardening metal from the group composed of molybdenum, vanadium, titanium, tungsten, zirconium and chromium, and a-total of from 0.05 to 6 per cent of at least two of the elements from the following metals lead, tin, thallium, cadmium, and bismuth, to improve its machining properties, the balance being substantially aluminum. 1

' 5. An aluminum base alloy consisting of abou 5 per cent copper, 0.75 per cent silicon, 0.5 per 7 cent bismuth, and 0.5 per cent lead, the balance being aluminum.

' LOUIS W. KEMPF.

WALTER A.'DEAN. 

